The US spends about $7000 a year on health care for each person whereas Cuba spends about $250 - and yet life expectancy in the two countries is almost identical at about 77. Cuba, indeed, spends about the same amount on health as Swaziland, but it has almost twice the life expectancy (pdf). How can all this be? Is the US wasting trillions of dollars?

One obvious conclusion - which continues to be surprising to many - is that life expectancy doesn't have much to do with spending on healthcare. In a famous figure published in the 1950s Tom McKeown showed that deaths from tuberculosis in Britain had dropped dramatically long before an effective treatment had been developed. In his 1979 book The Role of Medicine McKeown argued that medical treatment has little impact on mortality. Much more important, he argued, were income, social conditions, and nutrition. I remember coming across his work as a medical student in Edinburgh in the 1970s and being astonished.

Many doctors were naturally offended by McKeown's conclusion and hit back. John Bunker, a former professor of anaesthesiology from Stanford and a friend of mine, argues that medicine has comparatively recently developed a wide range of effective treatments that have been proved to work in clinical trials. He calculates that perhaps a quarter of improvements in life expectancy could be explained by medical care.

There have since been many studies on why deaths from heart disease have fallen over the past 20 years in developed countries like the US and Britain. Public health enthusiasts argue that the fall has been caused by reduced levels of smoking and changes in lifestyle. Cardiologists think that it's their effective treatments - clotbusting drugs, bypass operations, statins, and the like. There seems now to be rough agreement that about three-quarters of the improvement comes from reductions in smoking and changes in lifestyle and about a quarter from medical care.

The fact that Cuba has a similar life expectancy to the US is only a little to do with healthcare. The main reason may be that Cuba has a small gap between the richest and the poorest, whereas the US has as large a gap as any country in the world. The rich in the US live perhaps a dozen years longer than the poorest. It's the same in Britain. But Richard Wilkinson has long argued that above a certain relatively small income it is the distribution of wealth in a community rather than its average income that determines life expectancy - and, as Oliver James argues, much else, including crime levels and educational achievement. The more evenly income is distributed the better a country and community does on all of these measures. This seems to be scientific support for socialism, and unsurprisingly the theory has been disputed.

But medical care is probably important in some way in explaining differences between Cuba and the US. One factor may be that the US has invested so much in healthcare that it has reached a point where increased expenditure produces not benefit but harm. Health economists think that increasing expenditure on health care produces real benefit - in morbidity more than mortality - until an expenditure of about $600 for each person. Thereafter the curve becomes relatively flat - meaning that more expenditure produces only small benefit - and at some point increased expenditure produces harm. Doctors begin to treat too many people - with small benefit but the inevitable harm that accompanies any operation or treatment. Famously, gynaecologists in the US increased their hysterectomy rates as their children reached college age and they needed bigger incomes. The institute of medicine estimates that around 100,000 people a year in the US are killed by medical error.

Another important factor may be the organisation of care in Cuba and the US. Despite its low expenditure on healthcare Cuba has a huge number of doctors - far more proportionately than Britain. Cubans have easy access to doctors, whereas the uninsured in the US have much greater difficulty accessing care. Despite being a doctor myself, I'm not convinced that more doctors would result in better care. Cuban doctors may be so effective because they are more like nurses than high powered specialists. The impact of healthcare on mortality doesn't come from heart transplants and expensive scanners but from simple treatments for blood pressure and the like, things that nurses do better than doctors. Indeed, the single most effective thing that a clinician can do for a patient is to help them stop them smoking.